1. Technical Field
The present invention relates to a reactor utilizing a reaction member comprising an electrolyte portion containing solid electrolyte, a fuel electrode portion arranged integrally with the electrolyte portion and contacting a fuel gas to react the fuel gas, and an air electrode portion arranged integrally with the electrolyte portion and contacting an oxygen-containing gas to react the oxygen-containing gas.
2. Background Art
Conventionally, as a reactor utilizing the above-described reaction member, a solid oxide-type fuel cell (Solid Oxide Fuel Cell: SOFC), a fuel gas reforming apparatus, and the like are known. For example, the SOFC described in a Japanese Patent Application Laid-Open (kokai) No. 2004-342584 will be explained below.
In the SOFC described in the above Japanese Patent Application, as the reaction member, a sheet body formed by stacking and firing a solid electrolyte layer as the electrolyte portion, a fuel electrode layer as the fuel electrode portion formed on the upper surface of the solid electrolyte layer, and an air electrode layer as the air electrode portion formed on the lower surface of the solid electrolyte layer is used. In this SOFC, a flat-plate stack structure, in which sheet bodies and metallic separators supporting the sheet bodies are stacked in alternating layers, is adopted.
For each sheet body, a fuel channel, through which a fuel gas (e.g., hydrogen gas) flows, is formed and defined in a space between the separator adjacent to the upper side of the sheet body (upper separator) and (the fuel electrode layer in) the sheet body, and an air channel, through which an oxygen-containing gas (e.g., air) flows, is formed and defined in a space between the separator adjacent to the lower side of the sheet body (lower separator) and (the air electrode layer in) the sheet body.
In the SOFC described in the above document, for each of the sheet bodies, an electrically conductive member (specifically, metal mesh or the like) for electrically connecting the upper separator and the fuel electrode layer of the sheet body is often confined in the fuel channel. It can be said that the electrically conductive member on the side of the fuel channel is a member which is electrically connected with the fuel electrode layer (fuel electrode portion) to give and receive electrical power to and from the fuel electrode layer (fuel electrode portion). As a material for the electrically conductive member on the side of the fuel channel, nickel or the like is generally used.
Similarly, for each of the sheet bodies, an electrically conductive member (specifically, metal mesh or the like) for electrically connecting the lower separator and the air electrode layer of the sheet body is often confined in the air channel. It can be said that the electrically conductive member on the side of the air channel is a member which is electrically connected with the air electrode layer (air electrode portion) to give and receive electrical power to and from the air electrode layer (air electrode portion). As a material for the electrically conductive member on the side of the air channel, metal containing iron, chromium as a main component, e.g., stainless steel (specifically, ferritic SUS or the like) is generally used.
Thus, when the electrically conductive members are respectively confined in the fuel channel and the air channel, the SOFC described in the above document is assembled and manufactures, for example, as follows. First, in a state in which an adhesive interposes at the bonded surfaces between the sheet body and the separator adjacent to each other, the sheet bodies and the separators are stacked in alternating layers with the electrically conductive members interposed on the side of the fuel channel and on the side of the air channel respectively. During the stacking process, in order to secure the reliability of the electrical connection between the upper separator and the electrically conductive member on the side of the fuel channel, as well as the reliability of the electrical connection between the lower separator and the electrically conductive member on the side of the air channel, by means of welding, diffusion bonding or the like, the electrical connection portion between the upper separator and the electrically conductive member on the side of the fuel channel may be previously fixed in an electrically connected state, as well as the electrical connection portion between the lower separator and the electrically conductive member on the side of the air channel may be previously fixed in an electrically connected state.
In addition, during the stacking process, in order to secure the reliability of the electrical connection between the fuel electrode layer of the sheet body and the electrically conductive member on the side of the fuel channel, electrically conductive adhesive paste (e.g., nickel paste, or nickel oxide paste) may be previously coated on the surface of the fuel electrode layer of the sheet body. Similarly, in order to secure the reliability of the electrical connection between the air electrode layer of the sheet body and the electrically conductive member on the side of the air channel, electrically conductive adhesive paste (e.g., silver paste, electrically conductive ceramic paste, or platinum paste) may be previously coated on the surface of the air electrode layer of the sheet body.
After the assembly of the stack structure as described above, in order to solidify the above-described adhesive, and in order to solidify (fire) the above-described adhesive paste when the adhesive paste is coated, a heat-treatment is performed on the stack structure. Thereby, an SOFC having a stack structure is completed.
The above-described electrically conductive member (metal mesh or the like) made of stainless steel on the side of the air channel will be discussed below. The electrically conductive member made of stainless steel is likely to have chromia (Cr2O3) formed on the surface thereof at a high temperature of about 400° C. or higher and in the air. Accordingly, when a heat-treatment at 400° C. or higher is performed during the manufacturing (assembling) process of the stack structure as described above, it is likely that chromia is formed in the surface of the electrically conductive member. In addition, since the working temperature of SOFC is generally 400° C. or higher, chromia may be formed on the surface of the electrically conductive member during the operation of SOFC as well.
The chromia may be formed at the boundary between the electrically conductive member and the air electrode layer of the sheet body (i.e., on the surface of the electrical connection portion), when the electrically conductive member and the air electrode layer of the sheet body directly contact each other. Further, as described above, the chromia may be formed at the boundary between the electrically conductive member and the adhesive paste (i.e., on the surface of the electrical connection portion), when the adhesive paste interposes between the electrically conductive member and the air electrode layer of the sheet body. This is because air (oxygen) can be supplied to the boundary between the electrically conductive member and the adhesive paste through a large number of pores existing in the adhesive paste.
Chromia has a large electrical resistance. Therefore, when chromia is formed at the boundary between the electrically conductive member and the air electrode layer of the sheet body (or at the boundary between the electrically conductive member and the adhesive paste) (i.e., on the surface of the electrical connection portion) as described above, problems such as the increase in the electrical resistance (internal resistance) of the SOFC as a whole and the decrease in the output of the SOFC as a whole may occur.